Liquid crystal display apparatuses employ, as their light source such as a backlight unit, a surface light-emitting device which uses light coming from one or more light emitting diodes (LEDs) as a point light source and emits two-dimensionally-distributed light. Such a surface light-emitting device has a structure that there is provided a light guide plate with opposing main surfaces (upper surface and lower surface), light coming from one or more light emitting diodes enters one edge surface of the light guide plate, and the light outgoes out from one of the main surfaces (upper surface) of the light guide plate.
As a technology relating to the surface light-emitting device, Japanese Unexamined Patent Application Publication (JP-A) No. 2003-029262 discloses a surface light-emitting device having the following structure. In the structure, a protrusion is formed on a lower chassis (made of metal), and there is provided a notch on each of a reflection sheet and a light guide plate so as to be fitted with the protrusion. The reflection sheet and the light guide plate are mounted on the lower chassis in this order, and then, a light source is mounted on the lower chassis to be covered with the reflection sheet.
As for a surface light-emitting device of side light type using point light sources as its light source, JP-A 2007-311327 discloses a surface light-emitting device having the following structure. In the structure, point light sources are attached on a light-source substrate, the light-source substrate is fixed on an L-shaped substrate-fixing member with adhesive, and the L-shaped substrate-fixing member is fixed on a lower chassis with a screw. In this surface light-emitting device, when the screw is removed, the light-source substrate and the substrate-fixing member can be detached from the surface light-emitting device by being pulled out through the lower surface of the surface light-emitting device.
Further, JP-A No. 2011-108366 discloses a surface light-emitting device having the following structure. In the structure, point light sources are attached on a light-source substrate, the light-source substrate is fixed to a lower chassis by being pressed by a light guide plate toward the lower chassis, and the light guide plate is fixed to an inner chassis with a light-guide-fixing member. In this surface light-emitting device, when the light-guide-fixing member is removed, the light guide plate becomes movable and the light-source substrate can be taken out through an opening formed on a side surface of the surface light-emitting device.
In order to increase the display-screen brightness of the above-described surface light-emitting devices, it is required to increase the number of light emitting elements for increasing the density of the elements, or to increase the current to be supplied to each of the point light sources for enlarging the light fluxes emitted from the light sources. However, the both methods make the temperature around the point light sources high because of heat generated when the point light sources emit light, which results in a decrease in the light fluxes emitted from the point light sources and a decrease in the life of the point light sources, and further results in a decrease in the brightness and reliability of the surface light-emitting device.
In view of this matter, there have been proposed various methods to enhance the heat radiation of the devices. However, those methods provide various problems. For example, a device using such a method does not have a structure that light sources can be replaced easily. As another example, a greater space is required between a light guide plate and point light sources in order to make the replacement of light sources easier, which results in deterioration of the brightness. As another example, the thickness of the surface light-emitting device increases in order to make the replacement of light sources easier. Further, because the point light sources emit light in various directions at light-emitting angles from 0° to 180°, an increase of a space between the light guide plate and the point light sources enlarges the amount of light which does not enter the light guide plate but enters there after the light has been reflected by members located above or below the light guide plate. Therefore, such a structure can make a problem that a loss of reflection light is caused and the loss results in reduced brightness of the device.
For example, the surface light-emitting device disclosed in JP-A No. 2003-029262 has an advantage in heat radiation because the light source is directly attached to the lower chassis, but the replacement of the light sources is difficult because the light source is covered with a reflection sheet.
Further, problems which can be caused in JP-A Nos. 2007-311327 and 2011-108366 are described with reference to FIGS. 16A, 16B and 17A to 17D below. FIGS. 16A and 16B are a perspective view and a sectional view illustrating a structure of a conventional surface light-emitting device disclosed in JP-A No. 2007-311327. FIGS. 16A and 16B illustrate upper chassis 102, lower chassis 103, second lower chassis 104, point light sources 106, light-source substrate 107, light guide plate 108, reflection sheet 109, optical sheet 110 and screw 111. FIGS. 17A to 17D are a front view, a side view, a sectional view taken along the XVIIA-XVIIA line and a sectional view taken along the XVIIB-XVIIB line, illustrating a structure of a conventional surface light-emitting device disclosed in JP-A No. 2011-108366. FIGS. 17A to 17D illustrate lower chassis 103, inner chassis 105, point light sources 106, light-source substrate 107, light guide plate 108, reflection sheet 109, optical sheet 110, screws 111, light-emitting aperture 112, light-guide-plate-fixing member 113, and light-source-substrate-ejecting hole 114.
The surface light-emitting device disclosed in JP-A No. 2007-311327 has an advantage in heat radiation of point light sources 106 because light-source substrate 107 is directly attached to lower chassis 103 as shown in FIGS. 16A and 16B, but upper chassis 102 or lower chassis 103 can be deformed when light-source substrate 107 and upper chassis 102 which have fitted together are being separated, which makes deterioration of the workability. Further, the fitted members can be separated by moving the lower chassis 103, but the increased space between light guide plate 108 and point light sources is required, which results in deterioration of the brightness. The surface light-emitting device disclosed in JP-A No. 2011-108366 has an advantage in heat radiation of point light sources 106 because light-source substrate 107 is directly attached to lower chassis 103 and has an advantage in workability because light-source substrate 107 can be taken out through the hole of the side surface of surface light-emitting device 101 only by detaching light-guide-plate-fixing member 113 as shown in FIGS. 17A to 17D, but such a device has the structure that light-source substrate 107 is fixed by being pressed (in the direction of the arrow of FIG. 17D) with light guide plate 108, and requires a surface extending in the thickness direction of the device so as to make light-source substrate 107 and light guide plate 108 touch to each other on the surface. Therefore, it is difficult to decrease the thickness of the surface light-emitting device 101.
Consequently, there have been provided surface light-emitting devices not to satisfy all of the desired properties: an excellent workability, high brightness, long life, and reduced thickness.
The present invention seeks to solve the problem.